In a downlink (“DL”) of an Orthogonal Frequency Division Multiple Access (“OFDMA”) type of system, power control usually receives limited gain, although power control is still considered in various standards. In general, without continuous DL power control, equal power allocation to all OFDMA data sub-carriers in the DL direction is a very common practice in both academia and engineering fields, especially for wireless high speed data services.
However, there are areas such as control channels, downlink map (“DL-MAP”) and uplink map (“UL-MAP”) in 802.16d/e or alike, where worst case condition modulation and coding allocation is often assumed in order for the DL-MAP and UL-MAP to cover the whole cell site. With constant DL power and equal power allocation for all the sub-carriers within OFDMA symbols, signals going through different channel frequencies are carrying the same power for sub-carriers with the same modulation schemes. In the DL frame, the DL-MAP and UL-MAP include Information Elements (“IEs”) for data bursts of all connections.
While these data bursts are allocated with different modulation and coding schemes based on their channel conditions, it is non-ideal for all the IEs in the DL-MAP and UL-MAP to be sent with the same modulation and coding scheme. This is because the channel conditions are different for different connections. There is a fixed portion of the overhead information within the DL-MAP and UL-MAP. For these fixed portions of the MAPs, worst case channel coding, modulation and repetition is appropriate. However, since the fixed and variable portions of the MAP contents are channel encoded into a single burst, reaching to different mobiles with different channel condition is not easily accomplished.
With an OFDM type system where single frequency reuse pattern is assumed, Partial Usage of Sub-Channels (“PUSC”) is also assumed where the sub-carriers are uniformly interleaved across the target bandwidth. The sub-carriers are uniformly interleaved across the target bandwidth with the same coding and modulation scheme such as QPSK and ½ rate encoding. The control channel signals that arrive at a mobile station from two base stations, which mobile station is at a cell boundary, interfere with each other with a rough Signal to Noise Ratio (“SNR”) about 0 dB. This is because the same sub-carriers are used for both base stations and they carry the same power output with similar path loss. For example, FIG. 8 illustrates a situation where the above referenced interference can occur. In FIG. 8, a wireless device 802 resides within an overlapped cell coverage region 804. As discussed above, the wireless device experiences interference when trying to communicate with one base station 806 because the other base station 808 is also transmitting at the same power with a similar path loss.
Therefore a need exists to overcome the problems with the prior art as discussed above.